The present invention relates generally to silent chain and sprocket power transmission systems. In particular, the present invention relates to silent chain and sprocket systems that are capable of transferring power from either side of the silent chain.
Silent chain and sprocket power transmission systems are typically used in automotive engine timing system applications. The timing system transfers power from an automotive engine's crankshaft to the engine's camshafts, which operate the engine's intake and exhaust valves. A timing system may also be used to drive other engine accessories.
A basic timing system consists of a chain, two or more sprockets, and a tensioner. As its name implies, a tensioner applies a tensioning force to the chain. This tension ensures that the chain cannot slip on the sprockets, and thus change the engine timing. The tensioner of a timing system is important because, unlike many chain and sprocket applications, a slip by the chain of a timing system will change the valve timing which could result in poor performance or even engine damage.
Automotive timing systems also differ from other chain and sprocket applications because noise created by timing systems can reach an unacceptable level. Roller chains create a loud impact noise when the roller element of the chain strikes the sprocket root area. Roller chain and sprocket drives for engine timing systems often generate unacceptable levels of noise. Silent chains attempt to duplicate the quieter sliding contact that characterizes gear-tooth engagement. Therefore, many timing systems now use a silent chain and sprocket drive to reduce the amount of noise generated as the chain engages the sprocket.
A conventional silent chain is typically formed of interleaved sets of inverted tooth links. Inverted tooth links of traditional silent chains used in timing systems form two teeth on one side of the chain that engage sprockets. A set or row of links is assembled by positioning several links alongside of or adjacent to each other. The links are connected by pivot means, which are usually round pins received in a pair of apertures of each link of the row. Adjacent rows of links are interleaved so that apertures of adjacent rows are aligned to receive a pivot means that joins adjacent rows and allows relative rotation of the rows. The direction from one joined row to the next is the chain direction. Silent chains having teeth on one side of the chain are incapable of driving sprockets on both sides of the chain. An example of a traditional silent chain used in a timing system application can be found in U.S. Pat. No. 4,509,323.
Due to the limited engine compartment space available for separate engine accessory drive systems, many automotive timing systems are also used to drive engine accessories, as well as camshafts. Because traditional silent chains are incapable of driving sprockets on both sides of the chain, long and convoluted serpentine arrangements may be required to drive engine accessories. The size and complexity of today's automotive timing systems can be minimized if sprockets could be driven on both sides of the timing chain.
Roller chain and sprocket systems are well suited for driving sprockets on both the front and the back sides of the chain, but as mentioned above, the noise characteristics of roller chain systems make them undesirable for timing system use. Furthermore, existing silent chain designs that are capable of engaging sprockets on both sides of the chain do so by providing links having teeth on both sides of the chain to achieve driving contact between the back-side of the chain and a toothed sprocket. An early example of such a back-drive chain is disclosed by U.S. Pat. No. 1,140,319. FIG. 1 shows a chain 54 in back-side driving contact with a toothed sprocket 56. Back-side teeth 58 engage the sprocket 56. However, when a conventional silent chain incorporating toothed back-side driving links similar to those in FIG. 1 is used with a back-side chain tensioner, the intermittent contact between the back-side link teeth 58 and the chain tensioner can result in wear and vibration problems.
In timing systems having a tensioner, the tops of the teeth on the back side of such a chain make intermittent contact with the shoe face of the tensioner. That intermittent contact causes wear, shortens the service life of the tensioner shoe, and also leads to vibration problems that may cause the chain to stretch as well as increase the system noise level.
U.S. Pat. No. 5,967,926 addresses the intermittent contact problem by adding flat-faced links to the back-side of a silent chain. To promote increased sliding contact between the back-side of the chain and a chain tensioner, as shown by FIG. 2, non-driving outer links 66 were added to the chain. The flat profile 68 of the back-side of these outer links 66 extends beyond the back-side link teeth 64. The flat profile 68 on the rear side of the chain facilitates sliding contact with the shoe faces of a tensioner and prevents the back-side teeth 64 from contacting a tensioner shoe face. Of course, these additional links also add to the weight and material costs of the chain. As shown by FIG. 2, a back-side driving chain link 60 may have a shallow back-side driving link tooth root 62 that limits engagement of a sprocket by back-side teeth 64.
There is a need for a silent chain and sprocket assembly for use with a chain tensioner that is capable of providing driving contact on both sides of the chain. Furthermore, there is also a need for a silent chain and sprocket assembly for use with a chain tensioner that provides improved system wear and vibration characteristics over current designs without adding to the chain's weight or material costs.